In the terminal equipment domain of telecommunications, radio paths are becoming increasingly important. Cordless telephones, mobile radio terminals and the wireless connection technology “RLL” (Radio in the Local Loop) or “WLL” (Wireless Local Loop) are well-known examples of this. With the aim of defining a standard for a high-performance and universal air interface, an air interface known as “DECT” (Digital Enhanced (formerly: European) Cordless Telecommunication) was defined at the instigation of European companies. The DECT standard is described in the documents ETS (European Telecommunication Standard) 300 175-1, . . . , Oct. 9, 1992 of the ETSI (European Telecommunication Standards Institute) and is known therefrom.
A DECT system allows up to 120 simultaneous connections between base stations and mobile units, which are furthermore understood to include not only mobile terminals but also, as for example in the wireless connection technology “Radio in the Local Loop”, stationary system components which communicate with a base station via an air interface and which include the functional scope of a mobile unit. As such, up to 10 frequencies between 1.88 and 1.90 GHz are available and up to 12 simultaneous duplex voice connections (time slots, voice channels) can be implemented per frequency.
Furthermore, interworking between DECT and “ISDN” (Integrated Services Digital Network) is specified in the DECT standard. Along with the time slots (channels) at 32 kbit/s (“Full Slots”) and 8 kbit/s (“Half Slots”) required for voice connections, time slots with a transmission rate of 64 kbit/s provided to support ISDN are also specified.
Base stations and corresponding mobile units are generally known which, for faster data transmission of, for example, 64 kbit/s or to support DECT/ISDN interworking, support transmission rates of both 32 kbit/s “Full Slots” and 64 kbit/s “Double Slots.” That is to say up to 6 channels with a transmission rate of 64 kbit/s, i.e. a maximum of 2 complete ISDN connections having two base channels “B-channel”, each at 64 kbit/s, and one control channel “D-channel” at 16 kbit/s, are provided.
“Scanning” of local radio devices to determine free and occupied channels, which is carried out by all DECT devices at least every 30 seconds as a background process, is known from the DECT standard. The information determined in this way is stored in a list known as the RSSI (“Received Signal Strength Indication”) list, where free channels with little interference are identified by high values and occupied channels with substantial interference are identified by low values, for subsequent selection. If a radio cell is supplied with radio resources by two base stations, or if these base stations partially cover a common radio coverage area (overlapping of radio cells), whereby the first base station supports only “Full Slots” and the second base station both “Full Slots” and “Double Slots”, a mobile station which supports both “Full Slots” and “Double Slots” and which, for example, requires the voice transmission service, i.e. a transmission rate of 32 kbit/s (“Full Slots”), requires a facility to determine which base station provides this service.
In terms of a universal telecommunications system “Universal Mobile Telecommunication System” UMTS, it is also conceivable for two base stations to supply two different telecommunications systems, for example a first base station supplies a DECT radio area and a second base station supplies a “Global System for Mobile” GSM radio area, whereby the radio coverage areas totally or partially overlap each other for the implementation of the UMTS. Mobile units which support both systems, therefore, similarly require a facility to distinguish between these base stations.
An object to which the present invention is directed is to indicate a method for controlling the selection of base stations in a cellular radio telecommunications system, in which the radio transmission resources made available in the radio telecommunications system, in particular in an RLL or WLL system, are effectively used.